suggest &str.chars() on attempt to &str.iter()

check if `String` or `&String` or `&str`

Update compiler/rustc_typeck/src/check/method/suggest.rs

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

remove some trailing whitespace

src/test/ui/suggest-using-chars.rs

@@ -0,0 +1,7 @@
+pub fn main() {
+    let _ = "foo".iter(); //~ ERROR no method named `iter` found for reference `&'static str` in the current scope
+    let _ = "foo".foo(); //~ ERROR no method named `foo` found for reference `&'static str` in the current scope
+    let _ = String::from("bar").iter(); //~ ERROR no method named `iter` found for struct `String` in the current scope
+    let _ = (&String::from("bar")).iter(); //~ ERROR no method named `iter` found for reference `&String` in the current scope
+    let _ = 0.iter(); //~ ERROR no method named `iter` found for type `{integer}` in the current scope
+}

src/test/ui/suggest-using-chars.stderr

@@ -0,0 +1,48 @@
+error[E0599]: no method named `iter` found for reference `&'static str` in the current scope
+  --> $DIR/suggest-using-chars.rs:2:19
+   |
+LL |     let _ = "foo".iter();
+   |                   ^^^^ method not found in `&'static str`
+   |
+help: because of the in-memory representation of `&str`, to obtain an `Iterator` over each of its codepoint use method `chars`
+   |
+LL |     let _ = "foo".chars();
+   |                   ~~~~~
+
+error[E0599]: no method named `foo` found for reference `&'static str` in the current scope
+  --> $DIR/suggest-using-chars.rs:3:19
+   |
+LL |     let _ = "foo".foo();
+   |                   ^^^ method not found in `&'static str`
+
+error[E0599]: no method named `iter` found for struct `String` in the current scope
+  --> $DIR/suggest-using-chars.rs:4:33
+   |
+LL |     let _ = String::from("bar").iter();
+   |                                 ^^^^ method not found in `String`
+   |
+help: because of the in-memory representation of `&str`, to obtain an `Iterator` over each of its codepoint use method `chars`
+   |
+LL |     let _ = String::from("bar").chars();
+   |                                 ~~~~~
+
+error[E0599]: no method named `iter` found for reference `&String` in the current scope
+  --> $DIR/suggest-using-chars.rs:5:36
+   |
+LL |     let _ = (&String::from("bar")).iter();
+   |                                    ^^^^ method not found in `&String`
+   |
+help: because of the in-memory representation of `&str`, to obtain an `Iterator` over each of its codepoint use method `chars`
+   |
+LL |     let _ = (&String::from("bar")).chars();
+   |                                    ~~~~~
+
+error[E0599]: no method named `iter` found for type `{integer}` in the current scope
+  --> $DIR/suggest-using-chars.rs:6:15
+   |
+LL |     let _ = 0.iter();
+   |               ^^^^ method not found in `{integer}`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0599`.